Rejuvenation - Spent Hydroprocessing Catalysts - page 223

rejuvenation of spent catalysts using basic solutions can be achieved by optimizing leaching
parameters such as concentration of leaching agent, temperature, and contact time of leaching
solution with catalyst. A high selectivity for V leaching was also confirmed using a NaOH
solution [506] . The leaching was the most extensive during the initial contact of spent catalyst
with the solution, i.e., during the first 30min more than 80% of V was leached out. The
leaching efficiency was further enhanced by applying electric current.
7.2.3 Environmental and Safety Aspects
Corrosive and hazardous nature of both acidic and basic solutions used for the rejuvenation of
spent catalysts indicates on a much higher level of precautions and control required to ensure
the acceptable environment and safety of operation, compared with the rejuvenation using
aqueous solutions of organic acids. Chemical methods must be used to trap and neutralize
vapours released during leaching. Similarly, after extraction of metals from the leachate, a
chemical treatment of the latter, followed by removal of the remaining contaminants from the
water, must be performed before the reuse and/or disposal of water. For health protection, use
of special clothes and protective means by operators must be obligatory. The selection of
material for construction of the equipment used for rejuvenation is much more demanding
compared with the processes employing aqueous solutions of organic acids.
7.3 Solvent Extraction
A high solubility of coke with the incorporated contaminant metals in a solvent is an essential
requirement for this method to be attractive. In an ideal case, this technique may be applied at
the end of operation by replacing the flow of feed with a solvent. A removal of spent catalyst
would be necessary in the case that solvent extraction is carried out under supercritical
conditions. Some studies on solvent extraction appeared in the literature [495,507-509] ,
although commercial viability of this method is rather remote.
Potential applications of the solvent extraction for catalyst activity recovery were discussed by
Seapan and Guohui [507] , with focus on the solvents such as CO 2 ,SO 2 , and pyridine. Under
supercritical conditions, these agents exhibit lower viscosity and surface tension and higher
diffusivity compared with the normal conditions. Under such conditions even highly aromatic
materials can be displaced from catalyst surface and solubilized. The level of coke removal
was influenced by temperature, pressure, and duration of extraction. However, even for the
best case, only about 50% of the coke remaining on the catalyst surface after toluene
extraction could be removed. This increased pore volume from 0.17 to 0.22mL/g and surface
area from 106 to 137m 2 /g for the spent and extracted catalysts, respectively. There is little
information available on the removal of metals under similar conditions, although one study
indicated that less than 50% of Co could be removed from the spent CoMo/Al 2 O 3 catalyst